Widely known in the art is a method of downhole hydraulicking of mineral resources (U.S. Pat. No. 3,951,457) according whereto development holes are sunk from the surface down to a pay-out bed and casings are installed in the holes so that the casing shoes are at least 4-6 m above the face of each hole. An apparatus for hydraulicking mineral resources, consisting of a monitor located at the lower end of the apparatus and a pulp-lifting mans (commonly a jet pump) at the end face of the apparatus, is lowered into the casing so that the monitor is below the casing shoe. A liquid, pressure-fed into the hole, turns into a high-pressure jet on passing through the monitor and breaks the rock. The pulp which forms is lifted to the surface. In operation, the apparatus travels upwards and forms cavities the floors where slope downward toward the hole so that the pulp can flow to the jet pump.
The prior art method of downhole hydraulicking of mineral resources is ineffective in working water-bearing beds and those containing drift sand. In incompetent beds, the water jet has a short range and the yield of mineral from the cavities is low. As the mineral is being broken and removed from the face, a continuous caving in takes place in this case. The caving in spreads to the overlaying strata and reached the dead rock of the overburden with the result that this rock is lifted to the surface. Sagging of the ground surface is a hazard which is very likely to occur in practicing the prior art method of downhole hydraulicking of mineral resources when shallow pay-out beds are being worked or the overburden is an incompetent one. This creates a professional hazard and endangers the safety of the hydraulic mining equipment.